The present invention relates to a method of detecting breakage of an electrode of an electric discharge machine of the wire electrode type.
A typical example of a conventional system for detecting breakage of a wire electrode of an electric discharge machine is shown schematically in FIG. 1. As seen in FIG. 1, the system includes mechanical limit switches 1, 2 and 3 disposed along the path of a wire electrode 4 passing through a workpiece 5. Machining power is supplied through a pair of feeding dies 6 and 7 which are in contact with the wire electrode 4. The wire electrode 4 is supplied from a supply bobbin 10 through a brake mechanism 9 for controlling the tension in the wire electrode and is recovered in a recovery box 8. The limit switch 1 detects when the wire electrode 4 on the bobbin 10 is consumed.
In such a system, if the wire electrode 4 is broken in the vicinity of the workpiece 5 as shown in FIG. 2, the fact of breakage can be detected by the separation of the wire electrode 4 from the mechanical limit switches 2 and 3.
Since a system using mechanical limit switches utilizes the mechanical displacement of the wire electrode 4 to detect breakage of the wire electrode, it is possible to arrange these switches at any position along the path of the wire. However, the response time of a mechanical switch to wire breakage is considerably slow, potentially causing a problem to occur immediately after the breakage of the wire electrode 4, as exemplarily illustrated in FIG. 3. That is, there may be a case where the supply of power from a main power source 100 to the workpiece 5 and the feeding dies 6 and 7 continues for a relatively long time after the breakage of the wire electrode 4 occurs. It has been found that it takes typically 100 to 300 ms from the time of breakage of the wire electrode 4 to actuation of either or both of the mechanical limit switches, during which time discharge can occur several thousand times. A broken end of the wire electrode 4 may contact a position 20 of the workpiece 5 and, if the wire electrode 4 is separated from the feeding die 6 at a position 21, discharge may occur at the position 21, causing the feeding die 6 to be eroded abnormally. If such undesired discharge is repeated, the surface of the feeding die 6 may become too rough to assure acceptable electrical contact with the wire electrode 4 thereafter. According to an experiment, it has been found that the average service life of a feeding die is only several tens of hours when such wire breakage occurs repeatedly. This means that the service life of the feeding die is considerably shortened by such wire breakage, compared with the usual service life of several hundred hours.
Furthermore, the machining table (not shown) of the electric discharge machine may move during the time from the occurrence of wire breakage to the time the limit switch responds. Therefore, since the position of the table may be remote from the desired working position when a broken wire electrode 4 is repaired, the connected wire electrode 4 may not contact with the workpiece 5, and thus it may be impossible to restart the discharge machining operation without repositioning the table.
Thus, according to the conventional detection system using mechanical limit switches, the service life of the feeding die may be shortened and it is difficult to restart the machining operation if the table is stopped at an advanced position.